Non-volatile memories retain their stored information after their power supply has been turned off, and have many applications in digital information systems, control apparatus, and other electronic systems. Although several forms of non-volatile memories exist, each has serious shortcomings.
Over the last twenty years, much work has been performed developing magnetic bubble memories. However, bubble memories remain complex, and require large current drive for their operation. Additionally, the cost of bubble memory greatly exceeds that of other memory devices.
EPROMs and EEPROMs are capable of retaining information over extended periods of time without losing data, but the charge, and therefore the data, within an EPROM or a EEPROM will eventually be lost by thermally induced tunneling or leakage. Further, changing the information stored in an EPROM or a EEPROM cannot be performed rapidly. Static RAMs and dynamic RAMs while volatile, can be maintained by a battery during periods when the system power is turned off. However, batteries have a relatively short useful life, and thereby present the danger of losing valued information as the batteries become discharged.
While important strides have been made in improving the storage capacity of random access memories, it is still desirable to increase the storage capacities further, especially in non-volatile memories.
One potential solution to solving capacity problems in non-volatile devices is through the use of ferromagnetic gates. Such a ferromagnetic gate memory is disclosed in U.S. Pat. No. 4,931,428, assigned to the Assignee of the present application, which is incorporated by reference herein. The disclosed device is essentially a field effect transistor having a sheet of ferromagnetic material for its gate. The ferromagnetic material is electrically conductive such that an inversion layer can be created in the underlying channel of the field effect transistor, allowing current to flow between the pair of source/drain regions of the transistor. By passing current through the channel in a selected direction, a magnetic field is induced in the ferromagnetic gate having a specific orientation. Depending on the orientation and strength of the magnetic field, the current through the channel can be controlled through the deflection of electrons into and out of the higher resistance channel surface.
This device, however, has a relatively small modulation of resistance. By increasing the modulation of resistance, the sensing circuitry could be simplified, reducing the cost of the memory.
Thus, a need has arisen in the industry for a fast, non-volatile memory cell, having a high storage capacity and with an increased modulation of resistance.